如何在颤振小部件测试中捕获来自未来的错误？

Question

我在对在 Future 期间引发异常的小部件进行小部件测试时遇到问题。

重现问题的代码

这是一个简单的测试小部件，它以非常简单的方式重现问题（感谢 Remi Rousselet 对问题的简化）。

testWidgets('This test should pass but fails', (tester) async {
  final future = Future<void>.error(42);

  await tester.pumpWidget(FutureBuilder(
    future: future,
    builder: (_, snapshot) {
      return Container();
    },
  ));
});

预期结果

我希望测试能够顺利完成。相反，它失败并出现以下错误：

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The number 42 was thrown running a test.

When the exception was thrown, this was the stack:
#2      main.<anonymous closure> (file:///C:/Projects/projet_65/mobile_app/test/ui/exception_test.dart:79:18)
#5      TestWidgetsFlutterBinding._runTestBody (package:flutter_test/src/binding.dart:0:0)
#8      TestWidgetsFlutterBinding._runTest (package:flutter_test/src/binding.dart:577:14)
#9      AutomatedTestWidgetsFlutterBinding.runTest.<anonymous closure> (package:flutter_test/src/binding.dart:993:24)
#15     AutomatedTestWidgetsFlutterBinding.runTest (package:flutter_test/src/binding.dart:990:15)
#16     testWidgets.<anonymous closure> (package:flutter_test/src/widget_tester.dart:106:22)
#17     Declarer.test.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:test_api/src/backend/declarer.dart:168:27)
#20     Declarer.test.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:test_api/src/backend/declarer.dart:0:0)
#21     Invoker.waitForOutstandingCallbacks.<anonymous closure> (package:test_api/src/backend/invoker.dart:250:15)
#27     Invoker._onRun.<anonymous closure>.<anonymous closure>.<anonymous closure> (package:test_api/src/backend/invoker.dart:399:21)
(elided 17 frames from class _FakeAsync, package dart:async, and package dart:async-patch)

The test description was:
  This test should pass but fails
════════════════════════════════════════════════════════════════════════════════════════════════════

我的尝试

我已经尝试expect 错误，如果它不在Future 中，我会这样做：

expect(tester.takeException(), equals(42));

但该语句失败并出现以下错误

The following TestFailure object was thrown running a test (but after the test had completed):
  Expected: 42
  Actual: null

编辑：

@shb 答案对于暴露的情况是正确的。这是一个轻微的修改，可以打破它并返回初始错误。这种情况在实际应用中更容易发生（我就是这种情况）

  testWidgets('This test should pass but fails', (tester) async {
      Future future;

      await tester.pumpWidget(
        MaterialApp(
          home: Row(
            children: <Widget>[
              FlatButton(
                child: const Text('GO'),
                onPressed: () { future = Future.error(42);},
              ),
              FutureBuilder(
                future: future,
                builder: (_, snapshot) {
                  return Container();
                },
              ),
            ],
          ),
        ));

      await tester.tap(find.text('GO'));
  });

注意：我自愿省略了@shb 提出的tester.runAsync 以匹配最初的问题，因为它在特定情况下不起作用

Answer 1

用await tester.runAsync(() async { .. }包装你的代码

来自official documentationrunAsync<T>

运行一个执行真正异步工作的回调。

这适用于需要调用异步方法的调用者 其中方法产生隔离或操作系统线程，因此不能 通过调用pump同步执行。

见下文

testWidgets('This test should pass but fails', (tester) async {

    await tester.runAsync(() async {

      final future = Future<void>.error(42);

      await tester.pumpWidget(FutureBuilder(
        future: future,
        builder: (_, snapshot) {
          return Container();
        },
      ));

    });

  });

编辑：

（OP 提出的第二个问题）

在这种情况下使用

Future.delayed(Duration.zero, () {
   tester.tap(find.text('GO'));
});

下面是完整的sn-p

testWidgets('2nd try This test should pass but fails', (tester) async {

Future future;
await tester.runAsync(() async {
  await tester.pumpWidget(
    MaterialApp(
      home: Row(
        children: <Widget>[
          FlatButton(
            child: const Text('GO'),
            onPressed: () {
              future = Future.error(42);
            },
          ),
          FutureBuilder(
            future: future,
            builder: (_, snapshot) {
              return Container();
            },
          ),
        ],
      ),
    ),
  );

  Future.delayed(Duration.zero, () {tester.tap(find.text('GO'));});
});
});

编辑 2：

后来发现

Future.delayed(Duration.zero, () { tester.tap(find.text('GO')); });

没有被调用。

Answer 2

期货向其听众报告错误。如果Future 没有侦听器，它会通知Zone 未捕获的错误（src）。这是测试框架从中获取错误的地方。

克服此错误的一种方法是在错误 Future 之前等待侦听器。

  testWidgets('This passes', (tester) async {
    final Completer completer = Completer();
    await tester.pumpWidget(FutureBuilder(
      future: completer.future,
      builder: (_, snapshot) {
        return Container();
      },
    ));

    // has subscribers, doesn't inform Zone about uncought error
    completer.completeError(42);
    tester.pumpAndSettle();
  });

Answer 3

我认为问题在于您没有发现错误并导致应用程序崩溃。

我已经尝试捕捉错误并且测试通过了：

代码如下：

testWidgets('This test should pass but fails', (tester) async {
    Future future;

    await tester.pumpWidget(MaterialApp(
      home: Row(
        children: <Widget>[
          FlatButton(
            child: const Text('GO'),
            onPressed: () {
              future = Future.error(42).catchError((error) {});
            },
          ),
          FutureBuilder(
            future: future,
            builder: (_, snapshot) {
              return Container();
            },
          ),
        ],
      ),
    ));

    await tester.tap(find.text('GO'));
  });